1. Field of the Invention
The present invention relates, in general, to a method for preparing heteroepitaxial thin films and, more particularly, to the prevention of island structures from occurring in the heteroepitaxial thin films by use of grooved or curved substrates.
2. Description of Prior Arts
A thin film which is deposited on a different material substance with their lattices being of coherence, is called a heteroepitaxial thin film.
When materials are deposited on substrates by epitaxy, if the materials mismatch the substrates in lattice constant, that is, if the materials are larger or smaller in lattice constant than are the substrates, the lattice constant difference between the two subjects make coherent elastic strains existing in the formed heteroepitaxial thin films. It is well known by the teaching of Crashnamersh et al., that, in order to relax the elastic strains resulting from lattice mismatch, the epitaxial thin films get their surface to roughen, resulting in occurrence of island structures.
Now, it is recognized that the island structure of epitaxial thin films is one of the main factors which have a bad influence on the optical and electrical properties of devices such as semiconductor devices or electrical parts. In order to have superb photoelectric properties and exert a good interfacial reaction on other materials, epitaxial thin films are required to be deposited with maintenance of surface flatness. Therefore, the surface roughening mentioned above is the problem to be solved at any cost.
Since the time when Asaro and Tiller conducted the first theoretical research on the instability of the surface conformation, many researchers have studied on the microstructural change behaviors in heteroepitaxial thin films and reported the elastic strains which affect the behaviors. Must of the reports published thus far contemplate the causes of the surface roughening, for example, relations between elastic strain and stress and the resulting change in island structure, etc. However, there are found no reports regarding the techniques for preventing the causes so as to obtain planar heteroepitaxial thin films.
Typically, the suppression of the island structure is achieved by depositing a film at a critical thickness or less, lest a large quantity of elastic strain energy be accumulated in the film or by employing substrates which have similar lattice constants to those of the thin film materials. These suppression techniques, however, are not applicable where thin films are deposited at necessary thicknesses on various substrates desired, which more frequently occurs. Therefore, the conventional techniques are non-practical in most cases.
For instance, because of difficulty in achieving planar thin films upon the preparation of heteroepitaxial thin films, conventionally, tensional layers or defect annihilating grids are additionally used, as disclosed in U.S. Pat. Nos. 5,019,529, 5,238,869, 4,830,984 and 4,953,170. In addition, these prior arts are confined to the use of particular materials, for example, GaAs thin films deposited on silicon substrates.
Knowledge of the compressive stress behavior in the interface between a substrate and a deposited thin film allows the elastic strain energy existing in the thin film to be alleviated, leading to the present invention.
The intensive and thorough research on planar heteroepitaxial thin films, repeated by the present inventors aiming to enable thin films to be epitaxially deposited at a broad range of thicknesses on various substrates, resulted in the finding that the use of grooved or roughened substrates can inhibit the island structure from occurring in heteroepitaxial thin films without conducting any additional processes.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a method for preparing heteroepitaxial thin films which are free of island structures.
It is another object of the present invention to provide a method for preparing heteroepitaxial thin films, which enable the thin films to be flatly deposited at a significant thickness on various substrates without additionally processing.
Based on the present invention, the above objects could be accomplished by a provision of a method for preparing heteroepitaxial thin films, in which the heteroepitaxial thin films are deposited on grooved or curved surfaces of substrates.